Broadbanding adapter for circularly polarized antenna



June 11. 1968 A. c. VELDHUIS 3,388,400

BEOADBANDING ADAPTER FOR CIRCULARLY POLARIZED ANTENNA Filed May 28, 196E3 Sheets-Sheet l I'IGEI- I N VEN TOR.

A 43 GLWGZJ June 11, 1968 A. c. VELDHUIS 3,388,400

BROADBANDING ADAPTER FOR CIRCULARLY POLARIZED ANTENNA Filed May 28, 19653 Sheets-Sheet 2 IN VENTUR.

# EA- GA) ATTOHNEYJ June 11, 1968 A. c. VELDHUIS ,3

BROADBANDING ADAPTER FOR CIRCULARLY POLARIZED ANTENNA Filed May 28, 19653 Sheets-Sheet 5 V5 WI? I40 I50- mspwslvzw Me I N VEN TOR.

QJM 62A United States Patent 3,388,400 BROADBANDENG ADAPTER FORCIRCULARLY POLARIZED ANTENNA Albert C. Veldlruis, West Chester, Pa.,assignor to Trylon Incorporated, Elverson, Pa., 21 corporation ofPennsylvania Filed May 28, 1965, Ser. No. 459,545 2 Claims. (Cl.343--822) This invention relates to VHF (very high frequency) circularlypolarized antennae of the type or types in use for airport ground to aircommunications.

There are in operation a large number of circularly polarized ground toair control tower antennae of the Federal Aviation Agency. One exampleis FA-S 301 which were made in accordance with Federal Aviation AgencySpecification FAA-R-766b, Amendment No. 2 dated Apr. 27, 1960. Theseantennae utilize four half-wavelength dipoles spaced approximatelyone-third wavelength at the mid-band frequency of operation. Thefrequency range of the antenna is 118 to 135 megacycles. Each dipole isinclined 30 from the horizontal plane. The dipole elements of the fourdipoles are connected to a central support body or hub by means of fourparallel transmission lines of approximately 200 ohms impedance. Theinput receptacle at the base of the antenna is a type UG-S 8A/ U fittingfor connection to RG8/U coaxial cable.

The object of the present invention is to provide broadbanding meanswhich may be readily and easily installed in the field at the site ofexisting antenna type FA-530l, and similar type antennae, thereby toimprove the impedance characteristics of the antenna over a wider rangeof frequencies. This object is achieved in accordance with the presentinvention by the provision and connection of a simple broadbanding stub.

In the drawing:

FIG. 1 is a perspective illustration of a VHF circularly polarizedantenna, type FA-530l; included within the antenna mast 28, but notvisible in FIG. 1, is the broadbanding adapter of the present invention;

FIG. 2 is a view in section along the line IIII of FIG. 1 showing thestructural details of the broadbanding stub and the place and manner ofits insertion into an existing antenna type FA-5301 to transform theantenna to broadband operation; and

FIG. 3 is a graphical representation which illustrates the improvementachieved by the broadband device of the present invention.

Referring now to FIG. 1, there is illustrated a circularly polarizedantenna type FA-5301. This antenna has four half-wavelength dipoles 21,22, 23 and 24 each of which is inclined 30 from the horizontal plane.Each dipole is comprised of two elements identified by the suffix a andb added to the reference numeral of the particular dipole. For example,dipole 21 is comprised of dipole elements 21a and 21b; dipole 22 iscomprised of dipole elements 22a and 2212; etc. Each pair of dipoleelements is connected to a central support hub 25 by a ZOO-ohm paralleltransmission line. For example, dipole 21 is connected to the centralhub 25 by parallel transmission line 121 comprised of conductor arms121a and 121b connecting dipole elements 21a and 21b, respectively.

Central support hub 25 is comprised of an upper element 25a and a lowerelement 251) separated from each other by an insulating spacer 26. The adipole elements 21a, 22a, 23a, 24a, are connected to and supported bythe upper hub element 25a. The b dipole elements 21b, 22b, 23b and 24bare connected to and supported by the lower hub element 251;.

The upper hub element 25a, the four upper transmission lines 121a, 122a,etc., and the four a dipole elements, are all part of a one-piece solidaluminum alloy #43 cast- Patented June 11, 1968 ice ing. Similarly, thelower hub element 25b, the four lower transmission lines 121b, 122b,etc., and the four b dipole elements, are all part of a one-piece solidaluminum alloy #43 casting.

The lower hub casting also includes a depending cylindrical skirtportion 27 adapted to fit over the antenna mast 28 and to be securedthereto as by set screws 29. The lower hub casting includes, within thedepending cylindrical skirt portion 27 a depending tubular hub portion31.

As already indicated, the upper hub casting is separated from the lowerhub casting by an insulating spacer 26, preferably plastic. The upperhub casting is secured to the lower hub casting by four nylon studs 35each equipped with suitable O-ring 36, lock washer 34, and cap nut 37.

An antenna lead-in connecting rod 39 is threaded into a hole in theupper hub casting and extends downwardly along the center axis of thelower hub casting, within the depending tubular hub portion 31 fromwhich it is insulated by an acrylic tube 38.

Rod 39 terminates at its lower end in a pin connector which projectsbeyond the lower end of tubular hub 31 and is adapted to be received bythe center conductor socket of a coaxial-cable connector 42 (42).Secured to the lower end of the tubular hub 31 by screws 32 is a coaxialcable receptacle 41 type UG-58/A/U having a depending cylindrical flangeportion externally threaded for receiving the internally-threadedouter-conductor collar of the coaxial-cable connector 42 (42). The rod39 and its pin connector are suitably insulated from the receptacle 41.

In the existing antennae, type FA-530l, the connector 41 is normallyconnected to the cable connector 42 (42') of the coaxial input cable.However, to transform the existing antenna type FA5301 from narrow tobroadband characteristics, the coaxial input cable is, in accordancewith my present invention, disconnected from the receptacle 41 and abroadband adapter 60 is interposed between receptacle 41 and the inputend of the coaxial input cable. The structure of the broadband adapterand the manner in which it is connected will now be described.

To add the broadband adapter 60 to an existing antenna type FA-5301, theset screws 29 are loosened and the entire antenna mast assembly iscarefully lifted from the antenna mast 28, pulling and pushing thecoaxial input cable 49 up along with the antenna. Mast 28 will ordinarily be 0 2 /2 standard weight galvanized pipe with a nominal CD. of2.875". The collar portion of the connector, identified by the referencedesignation 42 (42), is uncoupled from the receptacle 41 and the pin ofthe antenna rod 39 is pulled out of the female socket portion of theconnector.

The broadbander device, identified comprehensively by reference numeral60, is now connected between the receptacle 41 and the connector, andthe connector becomes relocated lower down the mast 28. This connectoris seen at the bottom of the drawing in FIG. 2, and is identified as42'.

The broadbander device comprises a straight section of coaxial cable 61about which is coiled a length of helical coaxial cable 50 whichfunction as the stub. The coaxial cable section 61 and the helical stub50 are located within the bore of a section of tubing 131 of suitablematerial, such as brass, copper, silvered brass, etc. Tubing 131 has adiameter which allows it to be inserted into the antenna mast 28 and toabut against the lower end of the tubular hub 31 of the antenna lowerhub casting.

The upper end of the section of coaxial cable 61 is provided with aconnector 42 (similar to connector 42' referred to above) and adapted tocouple the center conductor of coaxial cable 61 to the antenna lead-inrod 39 3 and to connect the outer conductor of coaxial cable 61 to thetubular hub 31.

The lower end of the broadbander device 60 is provided with a receptacle141 similar to that of receptacle 41 located at the lower end of tubularhub 31. Receptacle 141 is secured to the lower end of the tubing 131 byscrews 132. The center conductor of coaxial cable section 61 terminatesat its lower end in a pin 161 which couples to the female inner portionof connector 42' of coaxial input cable 49.

At the lower end of helical coaxial stub 50, the center conductor of thestub is connected to the center conductor of the coaxial section 61, andthe outer conductor of the helical coaxial stub 50 is connected to abrass grounding ring 51. Ring 51 is secured to the tubing 131 by a setscrew 52. Thus, the outer conductor of the helical coaxial stub is ingood electrical contact with the brass tubing 131.

As seen in FIG. 2, the helical coaxial stub 50 does not occupy the fulllength of the coaxial cable section 61 and the annular space above thehelical stub is mostly occupied by a fiber insulator tube 63. The spacebetween the lower end of the fiber tube 63 and the upper end of thehelical stub 50 is filled with an insulating compound 64, such as DowCorning Compound DC-S.

The length of the coaxial section 61 is 8 inches. The point ofconnection of the helical stub 50 to the coaxial section 61 is 12 inchesfrom the dipole terminals at the location of the spacer 26. This isequal to .124 wavelength at 120 megacycles. The helical stub 50 is 34 /2inches long, which is equivalent to .350 wavelength at 120 megacycles.

I have discovered by the relatively simple expediency of inserting thebroadbanding device 60 just described to an existing circularlypolarized antenna type FA-5301, the antenna may be transformed from arelatively narrow band into a broadband antenna having a voltagestanding wave ratio versus frequency characteristic such as isillustrated in FIG. 3 by curve B.

Referring now to FIG. 3, curve A represents the voltage standing waveratio (VSWR) versus frequency of the existing antenna type FA5301 priorto insertion of the broadbander device 60. This antenna has a voltagestanding wave ratio of 1.7:1 measured at the end of a 50-foot length ofRG-8/ U coaxial cable. The frequency range is 118-135 megacycles.

I have found that the voltage standing wave ratio versus frequencycharacteristic of the existing antenna FA-5301 can be substantiallyimproved by the insertion of the broadbanding device 60. When the device60 is connected, the voltage standing wave ratio versus frequencycharacteristic becomes that shown by curve B in FIG. 3. Curve B, as willbe seen, represents a substantial improvement over curve A. In itsimproved form, the modified antenna has a voltage standing wave ratio of13:1 and a frequency range of from 118 to 144 megacycles.

Having described my invention, I claim:

1. A broadbanding adapter for modifying existing VHF circularlypolarized antenna operating in a frequency range which includes 120megacycles, said adapter comprising a straight length of coaxial cablehaving a coaxial open-end stub wound helically thereabout and connectedthereto near one end, and connector means for interposing said straightlength of coaxial cable and its helical coaxial open-end stub betweenthe input receptacle of the antenna to be modified and the connector atthe input end of the coaxial input cable, the length of said helicalcoaxial open-end stub being substantially .350 wavelength at 120megacycles and its point of connection being substantially .124wavelength from the antenna terminals.

2. A broadbanding adapter for modifying existing mastmounted VHFcircularly polarized antenna operating in a frequency range whichincludes 120 megacycles, said adapter comprising a length of conductivetubing adapted to fit within said antenna mast, and having within saidtubing a straight length of coaxial cable having a coaxial shuntopen-end stub wound helically thereabout and having the lower end ofsaid stub connected to said straight length of coaxial cable near thelower end thereof, connector means for connecting the upper end of saidstraight length of coaxial cable to the input receptacle of the antennato be modified, and connector means for connecting the lower end of saidtraight length of coaxial cable to the input end of the coaxial inputcable, the length of said helical coaxial open-end stub beingsubstantially .350 wavelength at 120 megacycles and its point ofconnection being substantiallly .124 wavelength from the antennaterminals.

References Cited UNITED STATES PATENTS ELI LIEBERMAN, Primary Examiner.

1. A BROADBANDING ADAPTER FOR MODIFYING EXISTING VHF CIRCULARLYPOLARIZED ANTENNA OPERATING IN A FREQUENCY RANGE WHICH INCLUDES 120MEGACYCLES, SAID ADAPTER COMPRISING A STRAIGHT LENGTH OF COAXIAL CABLEHAVING A COAXIAL OPEN-END STUB WOUND HELICALLY THEREABOUT AND CONNECTEDTHERETO NEAR ONE END, AND CONNECTOR MEANS FOR INTERPOSING SAID STRAIGHTLENGTH OF COAXIAL CABLE AND ITS HELICAL COAXIAL OPEN-END STUB BETWEENTHE INPUT RECEPTACLE OF THE ANTENNA TO BE MODIFIED AND THE CONNECTOR ATTHE INPUT END OF THE COAXIAL INPUT CABLE, THE LENGTH OF SAID HELICALCOAXIAL OPEN-END STUB BEING SUBSTANTIALLY .350 WAVELENGTH AT 120MEGACYCLES AND ITS POINT OF CONNECTION BEING SUBSTANTIALLY .124WAVELENGTH FROM THE ANTENNA TERMINALS.